Geothermal energy recovery systems, which apply geothermal energy to the production of power, are the object of a considerable amount of attention in the prior art (DiPippo, R. et al., Brown University, U.S.A., DOE, March 1980). While commercial production of geothermal fluids (e.g. for power production) out of a “classical” steam or water dominated reservoirs is pretty well at hand, such production from Hot Dry Rock (HDR) (or Hot Fractured Rock HFR)) or even from reservoirs with low permeability or insufficient water recharge by Enhanced (or Engineered) Geothermal Systems (EGS) is essentially in the research and development stage, due to many problems in their actual industrial application, including difficulties in sustaining circulation of fluids at a high enough rate at reasonable pressure and losses to the surroundings. Nevertheless, research and development of such systems are being pursed intensively since a large amount of energy is available in such low permeability, semi-dry or dry geothermal reservoirs (Jacquot, 1998).
In the prior art, geothermal energy recovery systems typical comprise production as well as injection wells, which are connected by a permeable geological layer or fractures. Re-injection of spent geothermal fluids has been stated as beneficial for management of geothermal resources (Sigurosson et al, Florence 1995, pp. 1967). Geothermal wells (production and injection) are built drilled vertically or at an angle slightly deviated from the vertical.
When using a vertical geothermal well for producing, viz. extracting, geothermal fluid from the underground, different geological layers having various levels of permeability are traversed. The production location in said vertical geothermal wells are made at the depth where the temperature is sufficiently high and optimum extraction of the geothermal fluid (water, steam or water and steam mixture) has been established using the slotted liner or open hole [Katagiri, 1970]. The produced fluid is typically a mixture of water and steam, and in the type of plants that is most common in the present art, the so-called “steam” plants, the fluid has a temperature higher than about 360° F. The steam is produced by flashing in the reservoir or in a surface flash tank. The wells may be artesian wells, i.e. self-flowing wells wherein the pressure at the bottom opening exceeds the accompanying hydrostatic pressure of the overlying fluid column in the well pipe or wherein there is a phase change within the well, and consequently geothermal fluid flows out at the wellhead. In the case of water dominated resources, a pump may be used to promote its flow to the surface in the absence of artesian action, or to increase the flow rate above that due to artesian action, or in order to suppress flashing within the well bore (Dipippo, R. et al., Brown University, U.S.A., DOE, March 1980 pp. 172-210).
On the other hand, re-injection wells are used to do away with spent fluids at the geothermal power plants like separated brine at the well heads (geothermal water being also called “brine” because of its mineral content), or blow-down from a cooling tower associated with a water-cooled condenser of a geothermal power plant. This is in addition to the natural replenishment of the geothermal resource mostly by meteoric water (e.g. from a rain source).
In the case of air-cooled binary power plants, viz. plants in which the extracted geothermal fluid does not directly generate power but transfers most of its heat to a working fluid (generally organic), essentially all of the geothermal liquid is re-injected into re-injection wells thus avoiding the depletion of the aquifer. The beat of the geothermal fluid, or a substantial part of it, is recovered by heat exchange with a working fluid, which is used in the production of energy in ways and by apparatus known in the art (see e.g. U.S. Pat. No. 5,598,706). The depleted geothermal fluid, viz. the geothermal fluid from which heat has been extracted, and which is still hot or at least warm, is preferably recycled as injection water into an injection well also connected to the permeable layer (see FIG. 1).
A prior art set-up by which geothermal fluid is extracted and re-injected is schematically illustrated in FIG. 1. Numeral 10 designates the ground level. Numeral 11 designates an injection well, vertical in this example, sometimes provided with an injection pump 12 by means of which water is injected into the well. Numeral 18 designates a permeable layer which is crossed by the injection well 11. The injection well is perforated at 14 to discharge the injected water into geothermal reservoir 13. The arrows 15 symbolic indicate the flow of geothermal fluid to a production well 16, from which the geothermal fluid exits as symbolically indicated by arrow 17. Alternatively, the above-mentioned wells can slightly deviate from the vertical, its location and orientation being determined by a natural fault or fractures (Albright and Dreesen, 1998).
Other geothermal energy resources are offered by the Xerolithic systems. (See for instance Nunz, 1993). In such resources the underlying rock does not spontaneously produce fluids at an adequate rate and at a useful temperature and it is necessary to artificially create or increase the permeability of the rock and to introduce therein heat transfer fluids. Such systems are called Hot Dry Rock (HDR) or Hot Fractured Rocks (HFR). Rather than exploiting geothermal resources which contain heat mainly in the rocks or contain relatively small amounts of geothermal fluid, and have low permeability, techniques were developed to induce fractures between the production and injection wells to allow larger contact area between the hot rocks and the geothermal fluid while reducing the resistance to flow (Salazar and Brown, 1995). FIG. 2 schematically Illustrates such a prior art set-up. The ground is indicated once again at 10, an injection well is indicated at 21 and is provided with a pump 22. Artificial fractures 28 have been created in the ground and connect injection well 21 with two production well 24. The wells are perforated as at 25 in correspondence to fracture 23.
In such prior art Hot Dry Rock (HDR)/Hot Fractured Rocks (HFR) set-ups the production of geothermal fluids is limited by the permeability of the layers and by their thickness, and in case of fractures by the width of the fractures. The injection pumps require energy to be operated and the amount of energy consumed depends on the permeability of the layer between the injection and the production wells, and in some cases becomes a prohibitive fraction of the energy produced. Other problems are created by the large losses of injected fluid and by the difficulty of producing fractures in the desired direction. Mainly for these reasons the results of the prior art Hot Dry Rock (HDR)/HFR have been disappointing, to the extent that no power plant exploiting them has been built so far, and their only use today is for limited experimental heat extraction.
It is apparent, in the inventor's opinion that the critical stage in the exploitation of low permeability/low water content reservoirs is the flow of geothermal fluid from injection wells to production wells. This invention particularly intends to improve said flow while reducing fluid losses and thereby the whole technology of the use of geothermal fluid for the production of energy in such reservoirs otherwise useless.
It is therefore a purpose of this invention to overcome, at least substantially, the drawbacks of the prior art in the stage of producing hot geothermal fluid from such geological formations.
It is another purpose of this invention to provide a new and improved method and apparatus for permitting to use the heat of geothermal fluids for the production of energy.
It is a further purpose of this invention to enhance the efficient flow of geothermal fluids from injection wells to production wells.
It is a still further purpose of this invention to apply the technology of generating horizontal wells for improving the flow of geothermal fluids from injection wells to production wells.
Other purposes and advantages of the invention will appear as the description proceeds.